FIG. 4 shows a circuit diagram of a prior art optical signal detection circuit in which reference numeral 1 refers to a light receiving element made from a photodiode or phototransistor, and 2 refers to an output terminal. A left half 11 of the diagram is a current-voltage conversion circuit including a differential amplifier AMP 1 and a resistor R, and a right half 12 of the diagram is a voltage comparator having a hysteresis and made from a resistor R and a differential amplifier AMP 2 supplied with a reference voltage (hereinafter called a "hysteresis comparator").
An optical signal is converted into an electrical signal by the light-receiving element 1 and is subsequently processed electrically. In general, when a photo diode is used as the light receiving element 1, optical-to-electrical signal conversion is performed using the nature that the diode current is proportional to the strength of light. However, since the diode current is very small, the detected current signal is amplified in a current-voltage converting circuit of FIG. 4. An optical current signal detected by the photo diode 1 is converted and amplified into a voltage signal of a sufficient level in the current-voltage converting circuit 11. Since the signal includes a noise component, it is converted into a digital signal without noises by the voltage comparator 12 having a hysteresis. That is, the prior art optical signal detection requires a current-voltage converter having a high gain and a hysteresis comparator, and this invites an increase in the number of circuit elements to form the detection circuit. Beside this, since the prior art circuit requires such a current-voltage converting circuit having a high gain to amplify a small light signal, the circuit itself becomes weak against electrical external noises or is apt to oscillate. Further, also when some sets of optical detection circuits are incorporated into a single IC, it is very difficult to remove varieties in sensitivity of respective detection circuits, and it is necessary to adjust gains of respective current voltage converting circuits. Therefore, the prior art circuit is not suitable for incorporating some such circuits into a single chip as an IC.
It is therefore an object of the invention to provide an optical signal detecting circuit having a small-scaled circuit arrangement and suitable for incorporating some detection circuits into a single IC.